For example, in a wind turbine generator, a wind turbine and a nacelle are installed in an upper part of a tower, the nacelle housing an electricity generator rotationally driven by the wind turbine. The nacelle is structured so as to perform yaw-slewing (slew on a substantially horizontal plane) with respect to the tower in accordance with a direction of the wind so that the wind turbine receives wind power from the front thereof.
Further, as the structure of causing the nacelle to perform yaw-slewing with respect to the tower, there has been used a slewing bearing, which is formed by combining an inner ring with an outer ring through a plurality of balls or rollers. One of the inner ring and the outer ring is fixed to the tower, and the other is fixed to the nacelle (JP2007-107411 A).
Meanwhile, also in a construction machine such as a power shovel and a crane, there is slewably installed an upper frame with respect to a track frame serving as a lower structure, the upper frame being provided with an operating seat and a counter weight. The slewing bearing has been used as such a slewing structure (JP2005-61574 A).
The slewing bearing includes an outer ring, an inner ring, and a plurality of rotors. The outer ring includes a rolling surface of rolling elements, which is formed along an inner peripheral surface. The inner ring includes a rolling surface formed in an outer peripheral surface, the rolling surface is opposed to the rolling surface on a side of the outer ring. The plurality of rolling elements roll between the outer ring and the inner ring while receiving load. It is possible to use balls or rollers as the rolling elements. In a case of using not the balls but the rollers, it is necessary to arrange the rollers in a cross-roller structure with respect to a rolling surface or to provide rolling surfaces in a plurality of rows and cause the rollers to tilt in different directions in the rolling surfaces, respectively, in such manner that the inner ring and the outer ring are not separated due to load.    Patent Document 1: JP2007-107411 A    Patent Document 2: JP2005-61574 A